Ok, I just started to feed my fish Ken's cichlid and New Life Spectrum food. All my lace rock and sand are turning brown ( like a brown algae) . Is it the fish food? I'm going to clean my tank and take away one the food to find out what one is causing the problem. It only started when I started to feed the the new food. Maybe someone has the same problem and could cut some of the time out in finding the cause. I've had the tank for about 2months, but just when I switch the food it started. I do 20% water changes and clean the filter too.

I have about 10 cichlid ( mainly peacocks ) that are about an inch or less. I have them in a 55 g tank. I think I need a pleco to help clean the tank.

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I wish I did, but I already cleaned it, so I found some pic of what it looked like. It just didn't get on the lace rock, but also on the intake, out take, sand( just the crush coral, but not the PFS ), and the glass

It looked pretty much like this.

How to I keep this from happening? Is it the food or is it something else?

That does look like diatom algae. Your tank is still fairly new so I can see how diatoms may be possible. Can you clean the tank and switch to a different type of food to see if the brown stuff comes back? If it comes back without feeding the new food, then you would know that it is the food causing it. I've personally never heard of food causing this kind of algae, but I guess it would be possible depending on the ingriedients.

Probably less about food and more about it being a new tank. Diatoms are pretty common about this stage in a new aquarium. What kind of substrate are you using? That can be 1 of the biggest factors... if it is anything with silica (sand especially)?

I'm using pool filter sand. It's a new tank, but the filter has been used for about 1 year. Should I not of used PFS it seems like everyone here used it. Ok, so I think we figured it out what it is, but what should be do to stop it.

My guess is it is the PFS. It is a good substrate, but has a lot of silica in it. Many people who use PFS as a substrate experience a diatom outbreak. It will run its course over a few weeks. There is not a whole lot you can do other than clean it off and be patient.